Underwater motors constitute core components that drive most mechanical devices operating in underwater environments. Any exposure to water may cause the underwater motors, which include electric and mechanical parts, to malfunction or degrade seriously. Therefore, watertightness of underwater motors is crucial to prevent such problems. Specifically, in the case of mechanical devices operating in deep water, high-pressure water is likely to flow in along the rotation shaft of underwater motors, greatly increasing the necessity to design a structure to prevent such inflow of high-pressure water.